What have you changed? (mandatory)

Handle table split operations asynchronously.

What are the type of the changes (mandatory)?

• Improvement

Does this PR affect documentation (docs/docs-cn) update? (optional)

No.

Benchmark result if necessary (optional)

Using TiKV
The test code as follows：

start := time.Now()
	for j := 0; j < 1000; j++ {
		sql = fmt.Sprintf("create table db%d.t%d(a int, b int, c int, d varchar(255), e varchar(255))", i, j)
		if _, err = db.Exec(sql); err != nil {
			log.Errorf("create table err %v", err)
		}
	}
	log.Infof("exec sqls take time %v", time.Since(start))

use the original code, the result as follows:
2018/05/18 17:37:16 tidb_create.go:35: [info] exec sql use db0
2018/05/18 17:41:09 tidb_create.go:44: [info] exec sqls take time 3m53.493631103s
2018/05/18 17:41:09 tidb_create.go:35: [info] exec sql use db1
2018/05/18 17:45:03 tidb_create.go:44: [info] exec sqls take time 3m53.914944715s
2018/05/18 17:45:03 tidb_create.go:35: [info] exec sql use db2
2018/05/18 17:48:57 tidb_create.go:44: [info] exec sqls take time 3m53.528751212s
2018/05/18 17:48:57 tidb_create.go:35: [info] exec sql use db3
2018/05/18 17:52:56 tidb_create.go:44: [info] exec sqls take time 3m59.223324563s
2018/05/18 17:52:56 tidb_create.go:35: [info] exec sql use db4
2018/05/18 17:56:56 tidb_create.go:44: [info] exec sqls take time 3m59.867498659s
2018/05/18 17:56:56 tidb_create.go:35: [info] exec sql use db5
2018/05/18 18:01:01 tidb_create.go:44: [info] exec sqls take time 4m5.327908179s
2018/05/18 18:01:01 tidb_create.go:35: [info] exec sql use db6
2018/05/18 18:05:09 tidb_create.go:44: [info] exec sqls take time 4m7.886515515s
2018/05/18 18:05:09 tidb_create.go:35: [info] exec sql use db7
2018/05/18 18:09:15 tidb_create.go:44: [info] exec sqls take time 4m5.60381973s
2018/05/18 18:09:15 tidb_create.go:35: [info] exec sql use db8
2018/05/18 18:13:32 tidb_create.go:44: [info] exec sqls take time 4m16.640030126s

use the new code, the result as follows:
2018/05/18 18:42:03 tidb_create.go:35: [info] exec sql use db0
2018/05/18 18:44:08 tidb_create.go:44: [info] exec sqls take time 2m4.89636175s
2018/05/18 18:44:08 tidb_create.go:35: [info] exec sql use db1
2018/05/18 18:46:14 tidb_create.go:44: [info] exec sqls take time 2m5.840172046s
2018/05/18 18:46:14 tidb_create.go:35: [info] exec sql use db2
2018/05/18 18:48:22 tidb_create.go:44: [info] exec sqls take time 2m7.847079946s
2018/05/18 18:48:22 tidb_create.go:35: [info] exec sql use db3
2018/05/18 18:50:32 tidb_create.go:44: [info] exec sqls take time 2m9.464254105s
2018/05/18 18:50:32 tidb_create.go:35: [info] exec sql use db4
2018/05/18 18:52:42 tidb_create.go:44: [info] exec sqls take time 2m10.56319663s
2018/05/18 18:52:43 tidb_create.go:35: [info] exec sql use db5
2018/05/18 18:54:57 tidb_create.go:44: [info] exec sqls take time 2m14.954220944s
2018/05/18 18:54:58 tidb_create.go:35: [info] exec sql use db6
2018/05/18 18:57:14 tidb_create.go:44: [info] exec sqls take time 2m16.526956612s
2018/05/18 18:57:14 tidb_create.go:35: [info] exec sql use db7
2018/05/18 18:59:33 tidb_create.go:44: [info] exec sqls take time 2m18.566134241s
2018/05/18 18:59:33 tidb_create.go:35: [info] exec sql use db8
2018/05/18 19:02:33 tidb_create.go:44: [info] exec sqls take time 3m0.43806444s